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Milestone is largely symbolic, but "it's telling us we should be very concerned"

It's a new record, but one scientists aren't thrilled about hitting.

A benchmark reading of heat-trapping carbon dioxide in Earth's atmosphere hit a fresh peak in the past two weeks, and it's expected to stay at or above a milestone level for some time.

Instruments on Hawaii's Mauna Loa observatory first recorded carbon dioxide levels above 400 parts per million last May, peaking at 400.5. This year, the seasonally fluctuating number has crossed 401 ppm three times this month and hit a record 401.6 on March 12, said Ralph Keeling at the Scripps Institution of Oceanography in San Diego.

That's a figure well above pre-industrial levels and perhaps not seen since before the rise of humanity.

"The daily value is higher than any daily value we had last year, and it won't stop," Keeling said. "We're in a period where it's continuing to climb."

Concentrations rise and fall seasonally as the ocean and growing plants absorb more CO2 in the spring and summer. But the highs recorded in the past week come two months earlier than last year's, and they may stay above the 400 level as long as July, Keeling said.

"I think we'll probably be hanging up over 400 parts per million here for a couple of months," he said. Within a couple of years, it will be above 400 for good, he added.

Daily readings at Mauna Loa started with Keeling's father, Charles, in 1958, when the count was less than 320 ppm. The numbers have not only continued to rise, but they've also picked up steam as carbon emissions from fossil fuel consumption have built up in the atmosphere.

The idea that those emissions are changing the Earth's climate is politically controversial, but generally accepted as fact by the overwhelming majority of scientists. The 400 figure is largely symbolic, but "it's telling us we should be very concerned," said Donald Wuebbles, one of the lead authors of the United Nation's latest comprehensive report on climate change.

"This is very unusual, and it's indicative of what humans are doing to our climate," said Wuebbles, a University of Illinois professor of atmospheric science.

It's been several million years since the Earth saw carbon dioxide levels as high has they are today, Wuebbles said. If current trends continue, "We easily could reach 600 parts per million at the end of the century," he said. "That would put us into the CO2 levels of the age of the dinosaurs, and that we know was a much warmer world."

The current rate of climate change is about 10 times faster than other historical shifts, he said.

The National Oceanic and Atmospheric Administration also tracks CO2 concentrations, and its figures are "very consistent" with the Scripps readings, said Pieter Tans, who runs NOAA's greenhouse gas measurement program. NOAA's daily readings ran above 400 for most of this week and one day last week, he said.

An increase in carbon dioxide concentrations "unprecedented" in the last 20,000 years has helped drive up average temperatures by about 0.6 degrees Celsius (1 degree Fahrenheit) since 1950, the U.N.'s Intergovernmental Panel on Climate Change reported in September.

By 2100, temperatures could increase by 2 to 3.7 degrees Celsius (3.6 to 6.6 Fahrenheit), the report concluded. The result is expected to be a world of rising seas and more intense storms and droughts.

The IPCC found increasing evidence that ice sheets are losing mass, glaciers are shrinking, Arctic sea ice and snow cover are decreasing and permafrost is thawing in the Northern Hemisphere. Tide gauges and satellite data make it "unequivocal" that the world's mean sea level is on the upswing.

"The real message is that we're still moving in the wrong direction," Tans said. "The CO2 problem hasn't gone away. It continues to get worse every year."